Weathering Atmospheric Flow
by David Pescovitz
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Tina Chow joined the UC Berkeley faculty in July of this year.
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According to the US Department of Commerce, about one-third of this country's gross domestic product from private industry, approximately $3.5 trillion, is climate and weather sensitive. Industries like agriculture, air travel, and tourism are highly dependent on accurate forecasts while weather patterns are also intricately tied to pollution and even national security. The problem is that the models used for weather forecasting are incomplete. UC Berkeley environmental engineer Tina Katopodes Chow is helping fill the gaps.
Weather forecasting models are numerical systems that take observational data about factors like atmospheric pressure, temperature, wind speed and direction, and humidity and crunch those numbers in computer simulations of the atmosphere. At the heart of these simulations are complex equations that describe fluid dynamics, or flow, in the atmosphere. The region to be modeled, ranging from one city to the entire planet, is overlaid with a grid containing points where the equations are computed. The solutions to those equations are then output as a weather forecast. Faster computers have enabled higher resolution models to be used. The trick though is that a forecast model is only as good as the algorithms, or parametrizations, it uses.
"I work on improving the algorithms that go into those models so that the forecasts can be more accurate," says Chow, a newly-hired professor in the Department of Civil and Environmental Engineering.
Chow's work focuses on the atmospheric boundary layer (ABL), the region a mile or so above the Earth that most affects life on the planet. As the resolution of forecasting models improves, it sometimes becomes more difficult to predict the flow of the ABL, Chow says. That's because the details of the terrain, everything from mountains to skyscrapers, can shift the flow.
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In earlier work, Chow created simulations of the atmospheric boundary layer over the steep terrain in the Riviera Valley, Switzerland. Below is a computer-generated representation of the same cross section with the flow results. The color contours indicate flow along the valley axis in meters per second. The red contour means the flow is going up the valley, away from the observer, while blue is down the valley axis, towards the observer. (Tina Chow photo) |
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"Solutions for solving the equations that govern fluid dynamics are developed in idealized worlds," she says. "But when you do environmental flow, you have rough terrain. Houses, trees, roads, and valleys have to be accounted for in some way." Recently, Chow began looking at flow in the Sierra Nevada Mountains in Owens Valley , California . Under certain weather conditions, wind flowing over steep mountains sometimes stirs up an intense form of turbulence called a rotor. The phenomenon can be disastrous for airplanes and can also kick up massive amounts of dust, contributing to air pollution. Indeed, the dry Owens Lake bed is thought to be the largest single source of tiny dust particles in the western hemisphere. How the rotors form is not well understood.
Next year though, Chow and her colleagues will participate in a large multi-institution effort to study rotor waves using advanced sensors, meteorological towers, and aircraft. She hopes the data gathered as part of the Terrain-induced Rotor Experiment (T-REX) effort will inform the development of better algorithms to represent these kinds of phenomena in weather models.
"This will bring the numerical and observational sides of the problem together," she says.
This movie shows a simulation of atmospheric flow through the Riviera Valley during the two hours following sunrise. As the sun moves across the sky over the course of a day, shadows cover different parts of the Valley, affecting the heating of the ground and thereby the wind flow, represented by the moving arrows. In the video, the shaded surface represents the incoming solar radiation. Black is no sunlight and white is bright sun. (courtesy the researchers) [movie .avi file]
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Chow is also studying the atmospheric boundary layer in urban environments, where tall buildings and heat rising from sun-baked asphalt and industrial facilities can affect the flow. To tackle this complex "bottom topography," Chow and her colleagues are developing methods to simulate flows down streets between buildings, "urban canyons" comparable to the valleys found in the Sierra Nevadas. The trick is determining the boundary conditions, where the model begins and ends. After all, the flow moving through New York City doesn't just start at, say, Times Square . So in order to create an accurate high-resolution simulation, the urban flows must be combined with more traditional mesoscale models that simulate meteorological phenomena that can be spread across hundreds of miles, such as a system of thunderstorms.
"Urban and mesoscale models have never been truly coupled together before," Chow says.
Modeling the flow in complex urban terrains could lead to more accurate weather forecasts, especially in cities with microclimates. But it may also provide valuable insight about how materials are transported through the urban environment, from plumes of pollution and ash to possibly even more dangerous substances.
"We'd like to be able to predict in real time the dispersion of contaminants, either accidental or intentional, so that the effects might be controlled," says Chow, who previously worked in this area at Lawrence Livermore National Laboratory. "If we can get the flow model correct, the list of applications beyond weather forecasting becomes huge."
Tina Katopodes Chow's home page
"Professor Tina Chow Joins CEE Faculty" (CEE News & Events, 7July 2005)
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